Smith attended Knoxville Catholic High School in Knoxville, Tennessee. He played on both sides of the ball (running back and defensive back) for the Knoxville Catholic Fighting Irish football team. As a sophomore in 2004, he helped Knoxville Catholic to unbeaten regular season and final 10-1 record. He rushed for 1,312 yards and had 446 receiving yards (32 catches) with 23 combined touchdowns as a junior in 2005 on a Knoxville Catholic team that ended up 11-2. He intercepted five passes, returning two for touchdowns, forced three fumbles, recovered two fumbles and also made 87 tackles (11 for loss) at safety slot. He earned Tennessee Class 3A first-team All-state honors as junior from Tennessee Sports Writers Association. He was one of the top 33 running backs on 2006 preseason basis by CBS Sportsline.com College Football Preview. He was rated as the 46th best overall prospect in Southeast on 2006 preseason basis by The Sporting News SchoolSports. As senior in 2006, he gained 1,340 yards rushing to go with 19 touchdowns, caught 23 passes for 453 yards and six touchdowns, and also made 61 tackles, two tackles for loss, two interceptions and caused two fumbles on defense. He also averaged 14 yards per punt return for team that finished 12-1 after unbeaten regular season and Tennessee Class 3A quarterfinal loss. He helped '06 Knoxville Catholic team average 37 points per game. He was named the 2006-07 Gatorade Football Player of the Year in the state of Tennessee.[1] He was named to Super Southern 100 team for 2006 by Atlanta Journal-Constitution as one of nine safeties. He was the 2006 American General Tennessee Mr. Football Back of the Year in Class 3A. He was named 2006 Tennessee Class 3A All-state defensive back by Tennessee Sports Writers Association and by Tennessee Football Coaches Association, East Tennessee High School Football Player of the Year for '06 by the Lawrenceburg (Tenn.) Quarterback Club. Also named to Nashville Tennessean Dream Team for '06 as a safety. He made four tackles for West squad in MSL Events' East Meets West all-star game in January 2007 in Orlando.

In addition to being a star in football, Smith was also a starter on the basketball team and was a standout track athlete, where he was primarily a jumper. He won the state meet in the high jump (6-6 or 1.98 m) and the decathlon, scoring 6230 points. He posted a career-best leap of 2.03 meters (6-8) in the high jump.[2] He also had personal-best leaps of 6.54 meters (21-4) in the long jump and 14.15 meters (46-4) in the triple jump.[3]

Considered a four-star recruit by Rivals.com, Smith was listed as one of top 250 players nationally, including seventh among Tennessee prospects and among top 25 nationally in athlete category.[4] He was rated as the 89th best prospect nationally by CSTV, and sixth nationally among safeties. Many scouts believed he could play offense or defense in college. He chose Notre Dame over scholarship offers from Tennessee, Auburn, and Alabama.

Smith attended the University of Notre Dame from 2007 to 2011, where he played for the Notre Dame Fighting Irish football team. He started in 47 of his 51 collegiate contests and was one of six fifth-year seniors on scholarship on 2011 Irish roster (David Ruffer, Mike Ragone, Andrew Nuss, Gary Gray and Taylor Dever are the others). He served as the Irish captain in 2011. He is the only player in Irish football history to register more than 200 tackles, 15 tackles for loss and 15 pass breakups in a career. He leads or tied for lead among active Irish players in just about every defensive category, including games (38, tied), games started (34), total tackles (214), solo tackles (134), pass breakups (18), passes defended (25) and interceptions (seven). His 15.5 tackles for loss are third most on team. He has recorded three and a half career sacks and forced one fumble. He did not play as freshman but practiced that season at safety. He moved to linebacker as sophomore and started nine games for Irish at strong-side linebacker. He started at both safety and linebacker in 2009.

After red-shirting as a freshman in 2007, Smith emerged as a star in 2008 as a sophomore. He first began his career at linebacker. He played in all 13 games, starting 9 for the Irish and started all but four contests (San Diego State, Michigan, Purdue and Hawai'i). He recorded 57 tackles, which included 8.5 tackles for loss, and 3.5 sacks. He ranked fourth on the team in tackles with 57 stops, including eight and a half tackles for a loss. He also recorded three and a half sacks on the year and had seven pass break-ups. He ranked fourth on the team in tackles, led the team in tackles for loss, tied for team-high honors in sacks and tallied second-most pass breakups. He recorded three tackles (all solo), including a tackle for a loss in his Irish debut vs. San Diego State. He made his first career start and tallied six tackles (solo, five assists), including one for a loss against Michigan State. He amassed three tackles (two solos, assist) vs. Stanford. He also recorded his first career rush on a successful fake punt for 23 yards. He tied then a career high in tackles with six (three solos, three assists) including a tackle for a loss at North Carolina. He registered five tackles (three solos, two assists) in victory at Washington, where he added his first two career sacks and also ran a fake punt 35 yards. He recorded five tackles (four solos, assist) including a tackle for a loss in his fifth career start vs. Pittsburgh. He amassed a team-high nine tackles (seven solos, two assists) including one for a loss at Boston College. He made two tackles (both assists) and assisted on a tackle for loss and a sack in the win against Navy. He registered seven tackles (five solos, two assists), a sack and two pass break-ups against Syracuse. He tallied three tackles (all solos) and added a pass breakup in regular season finale vs. USC. He was credited with five tackles (all solos) and had two pass breakups in Hawaii Bowl victory.

As a junior in 2009, Smith was one of only three defensive players to start all 12 games for the Irish, joining safety Kyle McCarthy and linebacker Brian Smith. He started the first six games at safety before moving to strongside linebacker for final six contests. He ranked third on team with 69 tackles and tied for third with six and a half tackles for loss, while also adding four pass breakups and forced one fumble. He tallied six tackles including five solo stops in the season opener against Nevada. He recorded a team-best 11 tackles at Michigan, including one and a half tackles for loss. He registered seven tackles against Michigan State. He then switched to linebacker against Boston College and responded with four tackles, one tackle for loss and one forced fumble. He tied career-high with two tackles for loss against Navy. He led the Irish with nine tackles at Pittsburgh, where he also added one tackle for loss and one pass breakup. He led front seven and was second on team with 11 tackles against Connecticut. He totaled 299:32 of playing time on defense and ranked 10th on team with 127 appearances on special teams.

In his senior year in 2010, Smith saw action in all 13 games. He was one of 11 players to start all 13 games for the Irish. He joined linebackers Darius Fleming and Manti Te'o, cornerbacks Gary Gray and Darrin Walls and defensive ends Kapron Lewis-Moore and Ethan Johnson as the only players on the defensive side of the ball to start all 13 games. He totaled 93 tackles, including one-half tackle for loss (1 yard), seven interceptions (54 yards) and seven pass breakups. He ranked second on the team in total tackles, second in solo stops and third in assisted tackles. He led the team in interceptions and pass breakups. He ranked fourth in the FBS in interceptions per game (0.54) and total interceptions (seven). He eclipsed 10 or more tackles in four different games (Michigan State, Stanford, Pittsburgh and Navy). He registered four tackles, including three solo stops, in the victory over Purdue in the season opener. He collected nine tackles, including six solo stops, and one pass breakup against Michigan. He recorded another pass breakup and 10 more tackles at Michigan State. He eclipsed 10 or more tackles for the second straight game with 11 total stops, seven solo, versus Stanford. He made a pair of solo tackles and collected first career interception in the victory at Boston College. He was all over the field in the victory over Pittsburgh, recording a career-best 13 tackles, registering a pair of pass breakups and collecting an interception. He posted another a 10-tackle game against Navy. He collected a pass breakup and six tackles, four solo stops, against Tulsa. He registered a highlight-reel interception and made seven tackles in the victory over Utah. He recorded a tackle for loss and totaled five overall stops in the triumph over Army Black Knights at Yankee Stadium. He made six overall tackles, including five solo stops, registered a pair of pass breakups and collected an interception deep in Irish territory with only 36 seconds remaining that clinched the victory over USC. He registered three interceptions in the first half alone in the Hyundai Sun Bowl victory over Miami. The three interceptions not only equaled a school record, but also equaled the Sun Bowl record, becaming the 14th Notre Dame player to accomplish that, the last being Shane Walton versus Maryland in 2002. The three interceptions was also an Irish bowl game record. He was selected by ESPN.com to its All-Bowl team. He recorded 820 snaps on defense, the most of any defensive back and second most of any Irish defensive player.

In 2011, Smith was accepted to a graduate program at Notre Dame allowing him to play another season.[5] In his final year in 2011, serving as team captain, he played in and started in all 13 games. He recorded 90 tackles including 3 tackles for loss.

Smith was drafted by the Minnesota Vikings in the first round, 29th overall in the 2012 NFL Draft, becoming just the 3rd Notre Dame player and 3rd defensive back chosen by the Vikings in the 1st round in team history.

Smith had a record-setting debut as a rookie. He started all 16 games and Wild Card playoff game. He tied Vikings rookie record with 2 interceptions returned for touchdowns and was 1 shy of overall club record for a season. He tied for 3rd in NFL with pair of INT-return scores and ranked 2nd among NFL rookies. He ranked 2nd on team with 129 tackles and led team with 87 yards on interception returns while matching the team-high with 3 interceptions. He set Vikings record with 16 starts by a rookie safety, passing previous high of 14 by Chuck Lamson in 14. He ranks 3rd in INT return yards by a Vikings rookie. He had 10+ tackles in 5 games on the season and season-high 12 stops at Green Bay Packers (12/2) and at St. Louis Rams (12/16). He notched lone sack at Houston Texans (12/23), catching Matt Schaub. On October 8, 2012, Smith was ejected after flailing against an official. On October 19, 2012, he was fined $15,750 for a horse-collar tackle on Robert Griffin III. On October 21, 2012, Smith had his first career interception and returned it for a touchdown against the Arizona Cardinals. On December 9, 2012, Smith had his third career interception and returned it for a touchdown against the Chicago Bears.[6] Earned All-Rookie honors.

In 2013, Smith averaged over 8 tackles per game, notching 66 in 8 games played. He returned to the field for the final 3 games of regular season, coming back vs. Philadelphia (12/15) in upset win over playoff-bound Eagles. The Vikings went 2-1 in final 3 games of season. He missed 8 games as he recovered from injury. He suffered injured toe vs. Carolina Panthers (10/13) and was placed on injured reserve for recall. He grabbed 2nd INT in consecutive games vs. Cleveland Browns (9/22). He snagged 1st INT of season at Chicago Bears(10/15) when he picked off Jay Cutler. He started season opener at Detroit Lions (9/8) and led DBs with 10 tackles.